Method for assembling die package

ABSTRACT

A method of assembling a multi-chip module does not necessitate more the one pass through a die attach machine. The method involves attaching a smaller die to a larger die without using a die attach machine. The larger die may be attached to a support structure using a die attach machine.

This is a continuation of prior application Ser. No. 09/767,160, nowU.S. Pat. No. 6,553,658 filed Jan. 22, 2001, which is a divisional ofprior application Ser. No. 09/386,623, filed Aug. 31, 1999, which issuedon Apr. 10, 2001 as U.S. Pat. No. 6,212,767.

BACKGROUND

This invention relates generally to assembling integrated circuit dieinto packages.

For a number of reasons, it may be desirable to package two die togetherin a single package. For example, the components provided in a separatedie may not be capable of being integrated in a single integratedcircuit. For example, different process technologies may be used in eachdie. In addition, basic incompatibilities between components on each diemay necessitate separate die fabrication. Once the die are fabricatedindependently, it may be desirable to package them in a single packageso that a single set of contacts may be used to connect with each of thedie.

The handling of die, which may be stacked, to form a composite creates anumber of difficulties. Predominantly, semiconductor assembly equipmentis adapted to produce packages containing a single die. Thus, techniquesare needed to assemble the die.

Conventional die assembly techniques may involve a pair of passesthrough a die attach machine. For example, a larger of two die to bestacked may be attached to a support structure in a die attach machine.Thereafter, the support structure may be run through the die attachmachine again to attach the smaller die to a larger die.

The repeated passes through the die attach machine may bedisadvantageous for a number of different reasons. For example, theassembly process is complicated by the multiple passes through the dieattach machine. In some cases the die attach process may be relativelyexpensive. In some cases the thermal budget may be impacted by the dieattach process.

Die attach machines usually use an organic-based adhesive to attach thesupport structure to the die or to attach one die to another die. Theadhesive may be applied in an uncured or soft phase between the surfacesto be joined. The adhesive is then cured through exposure to elevatedtemperatures for a specified time period. The carrier material istypically an epoxy resin or polyimide. The carrier provides the adhesionand mechanical strength along the bond line. The carrier may be filledwith metal particles in the event electrical and thermal conductivity isrequired or with non-metallic particles such as alumina, if the ultimatebond should be an insulator.

Three types of polymer adhesives that are often used in die attachprocesses are epoxies, cyanate esters and polyimides. Epoxies userelatively low cure temperatures (125-175° C.) and have moderate glasstransition temperatures (100-155° C.). However, epoxies vary in theirability to absorb and outgas moisture and other contaminants. Cyanateester adhesives use higher cure temperatures (300° C.) and haverelatively higher glass transition temperatures (240° C.). However,unlike epoxy adhesives, cyanate esters have low outgassingcharacteristics as well as low ionic contents. Finally, polyimideadhesives use high cure temperatures (greater than 400° C.) and likecyanate esters have a high glass transition temperature. A variety ofconductive fillers including silver, gold or copper and non-conductivefillers, such as alumina, diamond, glass fabric, silica or ceramic maybe used with the adhesive.

Thus, in addition the complexity inherent in the die attach process, insome cases the thermal exposure may be disadvantageous both to shallowjunctions in advanced devices and to the wire bonds or other elementswhose electrical performance may be affected.

Circuits that are susceptible to outgassing and moisture generally use ametallurgical attachment technique. Solder or metal eutectic alloys mayprovide good thermal conductivity but because of their electricalconductivity they may not be used if the backside of the die must beinsulated from the support structure. Solders and eutectic alloysgenerally use higher processing temperatures and thereby imposeconstraints on work flow. For example, if a gold-tin die attachoperation is scheduled after other chips in a multi-chip module havebeen wire bonded, the high temperatures used may degrade the wire bondsor degrade the electrical performance of the actual device.

Thus, for a variety of reasons, it may be desirable in making multi-chipmodules, to reduce the number of times that the module or any partthereof must be exposed to the die attach process.

SUMMARY

In accordance with one aspect, a method of assembling multi-chip modulesincludes using a die attach machine to attach a first, relatively largerdie to a support structure. A second, relatively smaller die is affixedto the larger die without using a die attach machine.

Other aspects are set forth in the accompanying detailed description andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow in accordance with one embodiment of thepresent invention;

FIG. 2 is a more detailed embodiment of a process flow shown in FIG. 1;

FIG. 3 is a greatly enlarged top plan view of a die positioned on awafer;

FIG. 4 is a greatly enlarged cross-sectional view of a smaller diepositioned on a larger die;

FIG. 5 is a greatly enlarged cross-sectional view of a multi-chipmodule;

FIG. 6 is a more detailed process flow for a process in accordance withthe more general flow show in FIG. 1;

FIG. 7 is a top plan view of a die positioned on a wafer tape;

FIG. 8 is a top plan view of a smaller die positioned on a larger diepositioned on a wafer tape;

FIG. 9 is a more detailed flow for another embodiment corresponding tothe more general flow shown in FIG. 1;

FIG. 10 is a top plan view of a larger and a smaller die in position ona wafer tape;

FIG. 11 is an enlarged, cross-sectional view showing a larger die thathas been die attached to a support structure; and

FIG. 12 is a greatly enlarged cross-sectional view of a multi-chipmodule.

DETAILED DESCRIPTION

A process for forming a multi-chip module, shown in FIG. 1, avoids theneed to expose the elements of the module to a die attach machine morethan one time. As used herein, a multi-chip module is intended to referto a combination of two die stacked one on top of the other, and coupledto a support structure. A support structure may be any one of a varietyof devices used in making multi-chip modules including leadframes,substrates, and laminate cores, as examples.

The process flow may include three basic steps. One step is to dieattach the larger die to the support structure (block 10). This step isconventionally done in a die attach machine. Another step is to affix asmaller die to the larger die without using a die attach machine, asillustrated by block 12. Finally, the composite of the smaller die overthe larger die over a support structure may be packaged as indicated inblock 14.

Two different sequences may be utilized to assemble the multi-chipmodule. In accordance with one sequence, the smaller die may be affixedto the larger die and then that subassembly may be die attached to asupport structure as indicated by the arrows A1 in FIG. 1. Thereafter,the assembly may be packaged as indicated by arrows B1.

In accordance with another sequence, initially, the larger die may bedie attached to the support structure. Thereafter, as indicated by arrowA2, the smaller die may be affixed to the larger die which has alreadybeen affixed to the support structure. Finally, as indicated by arrowB2, the assembly may be packaged.

In general, it may be advantageous, in some embodiments, if the smallerdie is sufficiently smaller than the larger die to enable both die to bewire bonded after the smaller die is attached to the larger die, withboth die facing upwardly.

A more detailed process, shown in FIG. 2, for implementing one of thesequences illustrated in FIG. 1, may begin by singulating a smaller diefrom a wafer as indicated in block 16. The smaller die may then beattached to the larger die as indicated in block 18. In accordance withone aspect of the invention, this attachment is done without using a dieattach machine. Referring to FIG. 3, a semiconductor wafer 25 is shownwith a smaller die 30 attached to an unsingulated, larger die 28 stillin wafer form.

The smaller die 30 may be attached to the larger die 28 without using adie attach machine, but using conventional securement techniques. Forexample, adhesive tape such as lead on chip (LOC) tape, adhesives thatdo not need to be cured at temperatures above room temperature, solderor other non-die attach machine based techniques may be used.

For example, double sided die attach tape may be situated between thesmaller and larger die. A suitable tape is Sumioxy 5120T adhesive, 75 to125 micrometers thick. Bond pads for coupling the external devices maybe exposed peripherally about the larger die 28 so as to be accessiblearound the smaller die 30. A pick and place machine may be used tolocate the smaller die over the larger die.

Thereafter, the larger die 28 is singulated as indicated in block 20 ofFIG. 2. FIG. 4 shows a cross-sectional view of the smaller die 30attached by a securement medium 33 to the larger die 28 after the largerdie has been singulated in accordance with the step illustrated by block20.

The stack of the smaller die 30 and the larger die 28 may then be dieattached to a support structure, as indicated in block 22 in FIG. 2.Similarly, FIG. 5 shows the subassembly of the smaller die 30 and largerdie 28 attached to a support structure 34 through a die attach 32. Thedie attach 32 may be a heat curable adhesive deposited bysyringe-dispensing, screen printing or stamping paste adhesive, asexamples. Alternatively, perforated film adhesives may be used as thedie attach. Thereafter, conventional packaging techniques may beutilized, as indicated in block 24 in FIG. 2, including encapsulation.

The die attach process may be otherwise conventional in all ways. Thus,depending on the adhesive that is utilized, the necessary time andtemperature may be determined, as will be apparent to those skilled inthe art. For example, a conductive adhesive may be formed as an epoxypaste with a silver filler. Its glass transition temperature is 127° C.Similarly, a nonconductive adhesive may be formed of epoxy paste with analumina filler. Its glass transition temperature is 85° C.

In accordance with still another embodiment, a flow for assembling amulti-chip module using only one pass through a die attach machine maybegin by locating a larger die on a wafer tape as illustrated at block40 in FIG. 6. The wafer tape, sometimes called a wafer dicing tape, is atape with high toughness used to hold wafers during the dicing process.An example of wafer tape is a polyvinyl chloride membrane with adhesiveapplied to one side and mounted in a frame.

Referring to FIG. 7, the larger die 28 is shown in position on theadhesive side of a wafer tape 48. The tape 48 includes an adhesivecovered polymer membrane 52 secured in a frame 50. The frame 50 isillustrated by a rectangular frame but other shapes includingring-shaped frames may be utilized as well.

The die 28 is held in position by the adhesive on the membrane 52. Whileonly a single die is shown in position on the wafer tape 48 in FIG. 7,one or more die may be positioned on the tape 48 at one time, in someembodiments of the present invention.

Referring back to FIG. 6, the next step is to attach the smaller die tothe larger die using a pick and place machine. A pick and place machinetransfers the die 28 from the wafer tape 48 onto the larger die 28, asindicated in block 42. A die attach material may be positioned betweenthe stacked die. The result, shown in FIG. 8, has a smaller die 30 ontop of the larger die 28 still secured to the wafer tape 48.

The stacked die are then die attached to a support structure asindicated in block 44. In the die attach machine, the die attach layersbetween the die and between the layer die and the support structure maybe simultaneously activated and cured. The resulting composite, shown inFIG. 12, may use die attach material as the layers 32. Thereafter, thecomposite is packaged as indicated in block 46 in FIG. 6.

In accordance with still another embodiment, a larger die 28 may belocated onto the wafer tape 48 as indicated in block 54 in FIG. 9.Thereafter, a smaller die 30 may be attached next to the larger die 28on the wafer tape 48 as indicated in block 56. Thus, referring to FIG.10, the larger die 28 may be positioned next to the smaller die 30 onthe adhesive bearing side of the wafer tape 48.

The larger die 28 then may be stacked on a support structure, asindicated in block 58 in FIG. 9, using a die attach machine. Theresulting composite is shown in FIG. 11. The larger die 28 is mounted ona die attach 32 over the support structure 34.

Next the smaller die 30 is mounted onto the larger die 28 (block 60 inFIG. 9). This may be done using the die attach machine. The composite ofthe die attach materials, the larger and smaller die 28 and 30 and thesupport structure 34 is then secured together in a die attach machine.The composite, shown in FIG. 12 includes die attach layers 32 betweenthe die 28 and 30 and the support structure 34. The composite is thenpackaged as indicated in block 62 in FIG. 9.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A method of assembling multi-chip modulescomprising: singulating a first, relatively smaller die; attaching thesmaller die to a second, relatively larger die without using a dieattach machine; singulating the larger die; and attaching the smallerand larger die to a support structure using a die attach machine.
 2. Themethod of claim 1 including using a pick and place machine to mount saidsmaller die on the larger die.
 3. The method of claim 1 whereinattaching a smaller die to a larger die occurs before using a die attachmachine to attach the larger die to the support structure.
 4. The methodof claim 1 including attaching the larger and smaller die to a supportstructure including attaching said die to a wafer tape.
 5. The method ofclaim 1 including attaching the smaller die to a second relativelylarger die before singulating the larger die.
 6. The method of claim 1including attaching the smaller die to the larger die using adhesive. 7.The method of claim 6 including using double sided die attach tape toattach the smaller and larger die.